From the piece at Conversations:
Forming a rich tapestry of functionality and tightly woven features, the Lumia Denim Update combines new enhancements from the latest Windows Phone 8.1 Update 1, with exclusive experiences for Lumia smartphones.... We always strive to bring new features to everyone, but some of which are hardware dependent and are only announced for the Lumia 930, Lumia Icon, Lumia 1520 and Lumia 830, such as the Lumia Camera.
- Lumia Camera – Replacing Nokia Camera, Lumia Camera is faster and more intuitive than before to make the best camera experience, even better.
- Speed – Pocket to capture speeds are superfast, and it only takes milliseconds going from shot to shot.
- Moment Capture – A long press of the camera button starts 4K-quality video recording at 24 FPS. With each frame at 8.3 megapixels, you can save individual frames as high quality still images.
- Rich Capture – Auto HDR and Dynamic Flash are now welcomed into the already-impressive feature set. Shoot first and adjust the camera settings later for that perfect photo.
- Improved image processing algorithms – Updated to give you clearer photos, even in lowlight.
- “Hey Cortana” Voice Activation - Gone are the days of pressing the microphone button to wake-up Cortana. Simply say “Hey Cortana” while the phone is idle and Cortana will activate.This is possible thanks to the SensorCore technology.
- Glance Screen – Compatible Lumia devices can now show more at a glance with Bing Weather, or Bing Health & Fitness data, in addition to time and date, notifications and application data.
Lumia Denim Update will ship with Lumia 830, Lumia 730 Dual SIM and Lumia 735. For all other Lumia Windows Phone 8 smartphones, the update will start rolling out during Q4 2014, following partner testing and approvals.
As the preface said, quite a bit of this is heavily device-dependent, so don't expect much of this to come in the Denim update to most existing Windows Phone 8.1 devices.
The Lumia Camera and fast camera modes do seem to imply an abandoning of the processor-intensive oversampling system introduced in the Nokia Lumia 1020, in favour of faster operation to compete with other industry flagships.
The Glance screen additions are probably the main addition to Windows Phone 8.1 Update 1 for most existing (pre-1520/930) devices. If you have a 920/925/1020 (etc.) then there's a good case for simply going with the Update 1 Developer Preview programme right now, since there's not that much extra that you'll get in Denim.
Lumia 1520 and 930/Icon owners will want to hang back for the official Denim imaging upgrades, I think. Comments welcome.